1. Field of the Invention
The invention relates to optical fiber launch controls for improving the bandwidth of large core fibers of relatively short communication links and more particularly to a method of making an optical coupling apparatus that limits the optical modes launched into such fibers.
2. Description of the Prior Art
Fiber-optical communication links, because of their ability to carry vastly more information than equivalent sizes of electrical wires, will be the "infrastructure" of the much talked about information highway. In optical communication, sources of light, such as lasers, are connected to optical fibers which carry the signal over both long and short distances to optical receivers. Optical fibers typically have a central region of a high index of refraction surrounded by a cladding region of a lower index of refraction. Optical signals launched into the fiber propagate along its length in a multiplicity of optical modes. A problem with long fibers such as 1 km or longer, is that the time taken for the transmission of a signal via the different optical modes through the fiber varies, so that a rapid sequence of individual pulses fed into the fiber at one end can be smeared out into a continuous stream at the other end and all information is lost. To prevent this, fibers are being made with a radial gradient of refractive index having a parabolic cross section of index. In this way, the velocity of the various modes is compensated to reduce transit time variations among the modes. The number of possible modes can also be reduced by making the fiber very thin, i.e., of dimensions comparable to the wavelength of the light.
For such long, thin fibers, it is assumed that all optical modes, described the by angle .theta. at which they propagate down the fiber, are equally coupled to near neighbor modes and so equally couple and mix as the modes propagate down the fiber. If one allows the light to propagate for a sufficiently long distance, such as a kilometer, down such a fiber, all of the modes should be mixed together and from then on the effective fiber bandwidth will not vary with launch conditions.
Most datacommunications applications, however, require links under 500 m, the majority of which are under 100 m in length. In this length regime, the standard theory predicts great variation in fiber bandwidth with launch angle, even for small angles.
Fibers used in these shorter links are typically of a large core design, such as a step-index optical fiber. These fibers, because of their large cross-sectional areas, have a smaller bandwidth than fibers with small cross-sectional areas. Large core step-index optical fibers are known to exhibit intermode dispersion due to various postulated optical mode propagation and diffusion mechanisms. There has been experimental work which shows a large variation in fiber bandwidth upon changing the source (LED or laser) or the launch optics. This variation in bandwidth with launch has relegated the use of large core fibers to rather low data rates because the details of the variation in fiber performance with launch are not understood. Thus, there is a need to understand the mode propagation in a large core fiber and from that understanding derive launch design rules and optical structures to achieve reproducibly higher bandwidth in large core, short link fibers.